Display devices employing electroluminescent display elements, such as organic light emitting diodes (OLEDs), have become a popular choice among flat panel displays. EL displays are used as television screens, computer monitors, portable electronic systems such as mobile phones and personal digital assistants (PDAs). An OLED is a light emitting diode (LED) in which the emissive electroluminescent (EL) layer is a film of an organic material which emit light in response to an electric current. This layer of organic material is situated between two electrodes. Generally, at least one of these electrodes is transparent. An EL display functions without a backlight. Thus, it can display deep black levels and can also be thinner and lighter than other flat panel displays such liquid crystal displays (LCDs).
An EL display panel may comprise a plurality of pixels spatially arranged in a form of a matrix with a plurality of rows and a plurality of columns in a display area. For a color EL display panel, each pixel may comprise several OLEDs each emitting light of a different color in the visible spectrum from which various colors may be composed. FIG. 8(a) shows schematically a typical pixel 810 for a color EL display panel that has three light emitting elements 812, each emitting red (R), green (G), and blue (B) light, respectively. The traditional organic material used for blue OLEDs has a relatively low luminous efficiency. Consequently, a relatively large electrical current is required in order for a blue OLED to emit light with a certain intensity. Operating an OLED at the large electrical currents would have the adverse effect of shortening the lifetime of the OLED.
Recently, a new material of emitting yellow-green light with a higher luminous efficiency has been reported. Table 1 summarizes the luminous efficiencies for four EL materials used in color EL displays. In this table, R stands for an EL material for red OLEDs, G stands for an EL material for green OLEDs, B2 stands for a traditional EL material for blue OLEDs, and B1 stands for a new EL material for yellow-green OLEDs. The 1931 CIE color space (x, y) parameters associated with the light emitted by these materials are also listed. As can be seen in Table 1, the new EL material B1 for yellow-green OLEDs has a luminous efficiency that is more than four times that of the traditional EL material B2 for blue OLEDs. The new material B1 may be used in addition to the traditional material B2 for producing blue light in a color EL display panel.
TABLE 1the luminous efficiencies for four ELmaterials used in color EL displaysMaterial1931 CIE (x, y)Luminous Efficiency (cd/A)R(0.67, 0.33)35G(0.21, 0.70)90B2(0.14, 0.06)5B1(0.114, 0.148)22.5
FIG. 8(b) shows a pixel 820 for a color EL display panel that includes four light emitting elements 822, one emitting red light (R), one emitting green light (G), one emitting yellow-green light (B1), and one emitting blue light (B2). The four light emitting elements 822 are spatially arranged as a 2×2 sub-array.
FIG. 9 shows schematically an EL display panel 900 that has such a pixel arrangement as FIG. 8(b). The EL display panel 900 includes a plurality of pixels 910 spatially arranged in a form of a matrix with a plurality of rows and a plurality of columns. Each pixel 910 includes four light emitting elements 912 labeled as R, G, B1, and B2, respectively, according to the color of the light emitted by the elements. The EL display panel 900 includes a plurality of gate lines GAn and GBn (n=1,2, . . . ) and a plurality of data lines SAn and SBn (n=1,2, . . . ). Each gate line GAn is coupled to the R and B1 elements of a respective row of pixels, and each gate line GBn is coupled to the G and B2 elements of a respective row of pixels. A gate driver outputs scanning signals to the plurality of gate lines GAn and GBn sequentially so that the elements in the pixels are sequentially driven in a row-by-row fashion. Each data line SAn is coupled to the R and G elements of a respective column of pixels, and each data line SBn is coupled to the B1 and B2 elements of a respective column of pixels. Therefore, in this configuration, two gate lines GAn and GBn are required for each row of pixels and two data lines SAn and SBn are required for each column of pixels. For example, for a display panel with a 240×320 resolution, there would be 480 total data lines and 640 total gate lines. In comparison, for a display with the same resolution but comprising RGB pixels as shown in FIG. 8(a), the required data lines and gate lines would be 720 and 320, respectively. Therefore, the display panel 900 requires twice the number of gate lines as that required in a display panel with the same resolution comprising RGB pixels. Consequently, the scan driver is more complicated which in turn results in higher cost.
Therefore, a heretofore unaddressed need exists in the art to address the aforementioned deficiencies and inadequacies.